Dwarf Planet - Planetary-mass Moons

Planetary-mass Moons

Nineteen moons are known to be massive enough to have relaxed into a rounded shape under their own gravity, and seven of them are more massive than either Eris or Pluto. They are not physically distinct from the dwarf planets, but are not members of that class because they do not directly orbit the Sun. The seven which are more massive than Eris are Earth's moon, the four Galilean moons of Jupiter (Io, Europa, Ganymede, and Callisto), Titan of Saturn, and one moon of Neptune (Triton). The others are six moons of Saturn (Mimas, Enceladus, Tethys, Dione, Rhea, and Iapetus), five moons of Uranus (Miranda, Ariel, Umbriel, Titania, and Oberon), and one moon of Pluto (Charon). The term planemo ("planetary-mass object") covers both dwarf planets and such moons, as well as planets. Alan Stern considers them a special category of planets, "satellite planets".

Despite its requirement that dwarf planets orbit the Sun directly, draft resolution (5) presented to the IAU, which considered dwarf planets to be a category of planet, stated Charon could be considered a planet because it revolves with Pluto around a common center of mass located between the two bodies (rather than within one of the bodies). Note, however, that the Jupiter–Sun centre of mass lies outside the Sun, and so such a definition would imply that Jupiter is not a satellite of the Sun and therefore not a planet. This definition was not preserved in the IAU's final resolution.